Although there are several processes available for producing (mercaptoalkyl)alkoxysilanes, each has significant limitations. In one process, alkali hydrogen sulfides are reacted with (haloalkyl)alkoxysilanes in methanolic medium at normal pressure to produce the corresponding (mercaptoalkyl)alkoxysilanes (GB 1 102 251). However, this procedure requires an unusually long reaction-time (96 h) to achieve a high rate of conversion and typically results in a poor yield.
As an alternative, (mercaptoalkyl)alkoxysilanes may be produced by reacting alkali hydrogen sulfide with suitable (haloalkyl)alkoxysilanes in the presence of a 10–100% molar excess of H2S (U.S. Pat. No. 5,840,952). However, on an industrial scale, this process has the disadvantage that highly toxic H2S has to be stored, metered and handled. In addition, the process is carried out in two stages and this results in a diminished space-time yield.
Another process for preparing (mercaptoalkyl)alkoxysilanes is by converting (haloalkyl)alkoxysilanes with alkali hydrogensulfide (NaSH) in polar, aprotic solvents (EP 0 471 164). A drawback of this process is that it uses a large quantity, at least 50 vol. %, of solvent, and this may be highly toxic, e.g., dimethylformamide. In addition, the high boiling-point of solvents such as dimethylformamide makes distillative reprocessing and purification of the reaction products more difficult.